1. Technical Field
The present application relates generally to cellular communication systems and, in particular, to a method for grouping and ungrouping omni-cells using a common PN (Pseudorandom Noise) offset of one channel of each of the grouped cells in order to reduce signal traffic resulting from excessive handoffs, thereby decreasing the load on BTSs (Base Station Transceiver Subsystems).
2. Description of the Related Art
In general, mobile communication systems such as PCS (Personal Communication Service) and CDMA (Code Division Multiple Access) systems include a plurality of BTS (Base Station Transceiver Subsystem) for serving mobile terminals located in corresponding regions, a plurality of BSCs (Base Station Controller), a plurality of BSMs (Base Station Manager System) for managing and controlling a plurality of BSCs and BTSs, a plurality of MSCs (Mobile Switching Center), and a plurality of HLRs (Home Location Register). A CDMA mobile communications system typically includes multiple access channels. Each of the multiple access channels are comprised of a given sinusoidal frequency which is combined with, and supports, a multiplicity of messages each using distinct PN offsets and sequences (i.e, spreading codes). In other words, each different sinusoidal frequency and its corresponding group of distinct spreading codes form a multiple channel carrier.
During operation, mobile stations should not suffer from communication disturbance when moving from region to region (i.e., cell to cell). Accordingly, when a mobile station is operating in an idle state, it continuously reregisters a plurality of parameters with the cellular system of the region (i.e., cell) in which the mobile station is currently located. Moreover, during a telephone conversation, communication between the mobile station and the BTS is managed by the mobile station, the BTS, and the MSC, so as to maintain a sufficient and efficient radio link.
In a CDMA system, a system can receive mobile transmissions from two or more BTSs at any given time. In addition, a mobile station can receive signals transmitted by two or more BTSs at any given time. Consequently, a handoff operation can occur from one BTS to another BTS, or from one antenna region to another antenna region in one BTS area. During a handoff operation, the success of call connection and the quality of voice information should not deteriorate. Upon receiving the appropriate communication signals, receivers and transmitters in the cellular communication system determine when transmission is imminent, which triggers them to establish a correct time reference for synchronization.
In order to achieve efficient synchronization at the time of synchronization (especially at the time of early synchronization of the system), it is desirable to utilize signals which exhibit maximum autocorrelation functions at a 0 time shift and very small autocorrelation functions at all other time shifts. For this purpose, specific code words may be stored in the memory of a transmitter and receiver. In addition, a binary shift register series generator (which is a relatively simple linear system) may be utilized to generate codes (i.e., binary sequences) which have sufficient autocorrelation characteristics. Specifically, a Pseudorandom pattern generator (or PN code generator), which is composed of n stage shift registers, can be employed to continuously generate n bit outputs of 2.sup.n -1 (except the case wherein all bits are zero). The resulting bit sequence is referred to as a "PN sequence" since it resembles random noise code (except, of course, to the individuals who know the choice of the number and taps of shift registers), but has a repeatable pattern. A PN sequence has very desirable autocorrelation properties since the maximum autocorrelation value of all the PN sequences is given at 0 shift and a reduced autocorrelation value at all the other time shifts in one cycle (i.e., one chip). Consequently, the power spectrum density for a PN sequence approaches white spectrum density as the series length increases.
In a CDMA system, the offset of a PN sequence (i.e., the PN offset) is utilized to expand the bandwidth of modulated signals so as to increase the transmission bandwidth. As stated above, the PN offset is also utilized to discern between BTSs associated with users who utilize the same transmission bandwidth (i.e., a multi-access channel of same frequency channel) in the CDMA system. In an omni-cell structure, one BTS covers a single cell region, and signals which are transmitted from the BTS are first multiplied by a PN offset and a user's long code before being transmitted.
Referring now to FIG. 1, a block/flow diagram illustrates assigning paths of modulator chip channel according to the prior art. As is known in the art, each BTS in the communication system includes a modulator chip which is located in a channel card within the digital unit (DU) of the BTS. In a CDMA mobile communications system, an orthogonal code spreading scheme using Walsh codes to spread I Q baseband signals (as shown in FIG. 1) is typically employed for user discrimination and spectrum spreading. Ideally, the orthogonality of the Walsh codes enables users or channels to be discriminated without interference. For each channel in the modulator of FIG. 1 (e.g., CHANNEL #1-#N), there are 3 paths in which the same input signal can travel. By way of example, referring to Channel #1, a signal is encoded by encoder module 10, bit interleaved by an interleaver module 20, scrambled by a scrambler module 20 with codes from a long code generator 40, multiplexed with multiplexer 50, and then transmitted through the 3 paths .alpha., .beta., and .gamma.. In the omni-cell structure discussed above, only the first path (i.e., the .alpha.path) is used.
In general, each cell is discerned by a unique PN offset which is transmitted through a pilot channel. When a user (mobile station) travels between different cells, a handoff operation is performed by changing the unique PN offset associated with the source cell to the unique PN offset of the target cell. In areas having a significant amount of signal traffic due to an increase in the use of mobile stations by many subscribers, the rate of call connection can decrease due to system overload caused by an increased number of required handoff operations.
One method which is used to reduce the system load in a "hot spot" region (i.e., spatially localized user communication overloads which do not normally occur within a particular cellular region) is the dynamic channel assignment (or "channel borrowing") method disclosed in U.S. Pat. No. 5,722,043 to Rappaport et al. entitled "Method And Apparatus Of Assigning And Sharing Channels In A Cellular Communication System." Specifically, a plurality of cells (each having a centrally located base station) are formed into N clusters. Each cell included in the N cluster is allocated a set of distinct channels, which are further divided into subgroups of carriers. Each subgroup of carriers of each cell corresponds to an adjacent cell in the N cluster. When all of the distinct carriers assigned to a cell are utilized, a carrier can be borrowed from a corresponding subgroup of carriers of an adjacent cell.
The "borrowing" method disclosed in Rappaport is not of CDMA format, and does not disclose a method for grouping a plurality of cells (in which the grouped cells operate similar to an omni-cell) by utilizing a common PN offset for each grouped cell. Indeed, Rappaport teaches a general method of dynamically assigning channels of cells in a FDMA system. In addition, Rappaport does not teach a method for reducing loads of BTSs due to increased soft handoff operations (which cause the channels generated from two BTSs to generate double signal traffic).